Two characteristics have been observed on the effects of daylight entering a building through a conventional fenestration as applied to lighting control. These two characteristics, skylight and direct sunlight, involve two components of the daylight that are present in the light that enters the fenestration. The skylight component normally can be utilized effectively to light the work plane with a minimum upset to human comfort. However, the direct sunlight component has adverse effects on human comfort in that it produces very strong light contrast and radiant heat, both objectionable to human comfort. For these reasons, curtains or blinds are normally included on the windows to eliminate the objectionable sunlight effects. Curtains with partial transparency may be drawn by the occupants, or venetian type blinds adjusted to redirect the sun's rays away from the work plane. However, once the direct sunlight has left the building's exposure due to the sun's reorientation, the occupants normally open their curtains or drapes as they prefer the human eye comfort present when the eye is allowed to look at both close and distant objects.
Under conditions where daylight enters a room, the useful light on the work plane from the skylight component of the daylight is greatest close to the windows, and as one moves into the room away from the windows, the light level on the work plane progressively diminishes based on the light dispersal characteristics of the room. Thus, when daylight is available to reduce the artificial lighting system power load, the artificial lights closest to the windows may be progressively dimmed to a lower level than those at a greater distance from the windows.